The goal of this project is to define the genomic changes that cooperate with t(15; 17) to cause progression of Acute Promyelocytic Leukemia (APL). To study this question, we developed a mouse model of APL. In mice that express both PML-RARalpha (PR) and RARalpha-PML (RP) in early myeloid cells, progression to APL is usually associated with the acquisition of an interstitial deletion of chromosome 2 (del 2), which also occurs in mice that develop radiation or benzene-induced AML. We propose to define the importance of del 2 for APL progression, as follows: Specific Aim 1: We will define the minimal reqion of chromosome 2 relevant for APL progression using high-resolution Comparative Genomic Hybridization and RNA Array techniques. We will precisely define del 2 in PR/RP mice using long oligomer arrays and comparative genomic hybridization (CGH). The same arrays will be used to search for microdeletions, and genes that are silenced in tumors that contain del 2. These regions will be the first to be resequenced in Aim 2. Specific Aim 2: We will define the genetic alterations within del 2 that contribute to APL progression. Total exonic sequencing of the most relevant genes within the del 2 interval will be performed in a series of tumors with and without del 2. If mutant genes are identified, their human homologues will be resequenced in human APL samples. Verified mutations will be assessed biologically in appropriate mouse models. Specific Aim 3: We will establish the relevance of del 2 for APL progression by recreating the deletion in mice using homologous recombination and Lox P-Cre technology. We will define the endpoints of the minimal deletion in Specific Aim 1, and target Lox P sites to its ends. We will remove the region from chromosome 2 in embryonic stem cells (or adult mice) with Cre recombinase, and make adult mice that are heterozygous for del 2. The del 2 mice will be intercrossed with hCG-PML-RARalpha mice to assess the contribution of del 2 to APL progression. Additional mutations will be made as required to further define the critical regions within del 2.